Studies using the outbred CD1 mouse strain have shown that paternal irradiation of F0 mice six to seven weeks prior to mating causes significant changes in F1 and F2 embryonic cell proliferation. These changes are revealed as a competitive cell proliferation disadvantage in chimera assays when the affected embryo is paired with a normal embryo in an aggregation chimera. This effect is transmitted to F1 and F2 embryos for 1.0 Gy of 137Cs gamma rays, does not decrease between the F0, F1 and F2 generations, and correlates with significant decreases in body weights, changes in basal levels of selected signal transduction protein kinase activities and in elevated basal levels of p53 protein in the F3 offspring. In this project, we ask whether the genetic changes underlying these heritable consequences of paternal F0 irradiation are more likely to decrease, remain unchanged, or to increase with successive generations. Specific Aim I will be to determine whether competitive cell proliferation disadvantage in successive generations will differ when the outbred CD1 strain is replaced with the syngenic C57BL/6 strain. We will conduct a paternal F0 irradiation using C57BL/6 males to reduce differences in expressivity and will evaluate the F1, F2 and F3 offspring in chimera assays. If competitive cell proliferation disadvantage does not decrease with successive generations we will hypothesize that the genetic changes that cause this heritable effect are unstable across generations. Specific Aim II will be to test the hypothesis that the nuclear changes that cause competitive cell proliferation disadvantage include "DNA damage". We will irradiate a paternal F0 cohort that is homozygous for a p53 null mutation which may (indirectly) result in reduced DNA repair. If this hypothesis is correct, we expect the p53 null mutation to increase competitive cell proliferation disadvantage in the offspring of paternal F0 irradiation. Specific Aim III will be to test the hypothesis that the nuclear changes that cause competitive cell proliferation disadvantage will increase in number across generations to produce successively greater changes in correlative endpoints, including protein kinase activities. This outcome would agree with the hypothesis that these heritable consequences of irradiating the paternal germline may become amplified by genomic instability across generations.